1. Field of the Invention
The present invention relates to a resistance adjusting circuit which performs automatic adjustment such that a resistance of a resistance circuit of a semiconductor integrated circuit is set within a prescribed range based upon an external resistor element, and a semiconductor integrated circuit.
2. Description of the Related Art
A semiconductor integrated circuit for transmission (transmitter LSI) and a semiconductor integrated circuit for reception (receiver LSI) configuring a high-speed signal communication system are connected to each other through a differential transmission line. The receiver LSI comprises a receiver circuit whose differential input terminals are connected to the differential transmission line. Each of the differential input terminals of the receiver circuit is connected with a terminal end resistor. The terminal end resistor is provided in order to make the impedance of the transmission line and the impedance of the input terminal of the receiver circuit to be equal, so that reflection of power or the like caused by impedance mismatching is prevented. Accordingly, it is important to make adjustment such that a resistance of the terminal end resistor is equal to the characteristic impedance of the transmission line. A resistance adjusting circuit is used to adjust the resistance of the terminal end resistor.
Generally, the resistance adjusting circuit is configured to include a reference voltage generating circuit, a resistance control circuit, a comparison voltage generating circuit including a replica resistor circuit, and a main body resistor circuit used as a terminal end resistor. The reference voltage generating circuit generates a reference voltage having a value corresponding to a resistance of an external resistor element which is a reference resistor connected to an external terminal of an LSI. Both the replica resistor circuit and the main body resistor circuit have substantially the same configuration, and resistances thereof are adjusted according to a control code signal outputted from the resistance control circuit. The comparison voltage generating circuit generates a comparison voltage having a value corresponding to a resistance of the replica resistor circuit. The resistance control circuit comprises a voltage comparator and an up/down counter, and compares a reference voltage and a comparison voltage with each other by the voltage comparator and adjusts such that the resistance of the replica resistor circuit and the resistance of the main body resistor circuit is set within a prescribed range, with the result that the resistance of the main body resistor circuit is set within the prescribed range.
A plurality of resistors in each of the replica resistor circuit and the main body resistor circuit is selected according to a control code signal and the selected resistors are connected in parallel so that the combined resistance is adjusted.
When an adjustment pitch of the resistance in the resistance adjusting circuit is set fine in order to increase an accuracy of the resistance of each of the replica resistor circuit and the main body resistor circuit, a difference between the reference voltage and the comparison voltage becomes small. When the difference of the voltages is small, a mismatch voltage of the voltage comparator must be made smaller than the voltage difference by increasing a gain of the voltage comparator in the comparison voltage generating circuit, resulting in that it becomes difficult to design a circuit for the voltage comparator.
Incidentally, Jpn. Pat. Appln. KOKAI Publication No. 2001-94048 discloses a semiconductor device provided with an output impedance control circuit having a function of being capable of adjusting an impedance of an output buffer so as to match an external resistor element.